This article establishes minimum requirements for the use of Temperature Transmitters in process plants. Main keywords for this article are Temperature Transmitters Design Notes. Temperature Transmitters References. Temperature Transmitter Definitions. Transmitter Requirements. Safe failure fraction.
Temperature Transmitters Design Notes – Instrumentation
Temperature Transmitters References
Reference is made in this standard to the following documents.
American National Standards Institute (ANSI) / The Instrumentation Systems, and Automation Society (ISA)
ANSI/ISA SP 50.02 Fieldbus Standard for Use in Industrial Control Systems, Parts 2-6, 1997.
American Petroleum Institute (API)
RP 550 Manual on Installation of Refinery Instruments and Control System
RP 551 Process Measurement Instrumentation
European Committee for Electrotechnical Standardization (CENELEC) / European Committee for Standardization (CEN) CENELEC/CEN 94/9/EC ATEX Directive, 1994
Temperature Transmitter Definitions
Configurable. A device or system whose functional characteristics can be selected or rearranged through programming or other methods.
Dangerous detected: A failure which has the potential to put the safety-related system in a hazardous or fail-to-function state and can be detected by online diagnostics.
Dangerous undetected: A failure which has the potential to put the safety-related system in a hazardous or fail-to-function state and can not be detected by online diagnostics.
Electromagnetic Interference (EMI). Spurious effect produced in circuits on elements of a device by external electromagnetic field.
Grounded / Ungrounded. The presence or absence of an electrical connection between the “low” side of the transducer element and the portion of the transducer intended to be in contact
with the test structure. Method of ungrounding shall be stated as “internally ungrounded” or “by means of separate stud”.
Safe Failure Fraction: Safe failure fraction is the ratio of the (total safe failure rate of a subsystem plus the dangerous detected failure rate of the subsystem) to the total failure rate of the subsystem. (Subsystem refers to the individual devices).
Safe detected: A failure which does not have the potential to put the safety-related system in a hazardous or fail-to-function state and can be detected by online diagnostics.
Safe undetected: A failure which does not have the potential to put the safety-related system in a hazardous or fail-to-function state and can not be detected by online diagnostics.
Transmitter. A device that senses a process variable through the medium of a sensor and has an output whose steady-state value varies only as a predetermined function of the process variable.
Transmitter Requirements
- Smart transmitters complying with ANSI/ISA SP 50.02 shall be used. In exceptional cases of process constraints, conventional 4-20 mA signal type is acceptable.
- The transmitter shall be of the two (2) wire type. Four wire type are acceptable in situations where two (2) wire type cannot be used.
- The transmitter shall provide linearized output with respect to the input temperature.
- The transmitter shall have EMI / RF and voltage surge protection.
- In case of sensor burnout, the temperature transmitter shall have a choice of either upscale or downscale, indication.
- The transmitter shall have electrical isolation between the input and output signals. This feature allows the use of grounded thermocouples. The isolation shall be at least 500 Vrms.
- Installation of temperature transmitters shall conform to API RP 550/551.
- The transmitter accuracy shall be ± 0.015 percent of the calibrated span, or 0.1°C whichever is greater. For application requiring highly accurate temperature measurement, the sensor shall be individually calibrated and the sensor transmitter combination as an assembly shall be matched by the vendor.
- Transmitter shall be configurable in the field through local and remote communication devices.
- The transmitter vendor to provide following data for the instruments to be used in Safety Instrumented System (SIS) loops:
a) Safe Failure Fraction (SFf) (Diagonostic Coverage Factor)
.- Safe detected (Sd)
.- Safe Undetected (SUd)
.- Dangerous detected (Dd)
– Dangerous Undetected (DUd)
b) Mean time to repair (MTTR)
c) Frequency of functional check requirements - If local display is specified, then an integral 4-1/2 or 5 digit LCD meter with configurable engineering units shall be provided.
- The transmitter shall be suitable for the hazardous area application /ATEX Directive as per ‘Specification Sheet’.
- Head mounted (Integral Sensor Mounting) temperature transmitters are acceptable provided the mouting location is free from vibration and temperature extremes.